The invention relates to a down-hole drilling motor and a method for directional drilling by means of said motor of boreholes in subsurface formations, in the search for valuable materials such as oil and natural gas.
During drilling of a borehole in underground formations it is frequently required to vary or adjust the direction of drilling. Such adjustment of the drilling direction is commonly carried out by a kick-off procedure during which procedure a smoothly curved borehole section is drilled to bring the borehole at the desired course.
Various tools are known in the art for carrying out kick-off procedures. A suitable kick-off tool is disclosed in U.S. Pat. No. 3,260,318. This known tool consists of a down-hole drilling motor of the Moineau type. Part of the housing is bent such that in the operative position of the motor in a borehole, the axis of rotation of the drill bit is inclined with respect to the local borehole direction. During drilling by means of said motor a curved borehole section will be drilled when the drill string--and consequently also the motor housing--is not rotated. This known motor, however, is not suitable for drilling straight borehole sections and for each kick-off operation the motor has thus to be mounted on the drill string which requires a time consuming roundtrip procedure.
A down-hole drilling motor for alternately drilling straight and curved borehole sections is disclosed in U. S. Pat. No. 3,667,556. In this known motor the bearing assembly that supports the output shaft is connected in a pivotable manner to the motor housing. By varying the angle of deflection between the housing and the output shaft straight and curved borehole sections can be drilled at will. Major disadvantages of this known motor reside in the fragility of the pivots between bearing and housing and in the complexity of the remotely controlled positioning system that is responsible for adjusting the angle of deflection.